Considering first the exemplary or illustrative application to such defect inspection, numerous prior systems and techniques have been evolved for scanning such wafers, circuit boards or other surfaces and analyzing the images produced by reflecting light from such surfaces, including mask comparisons with desired or "good" pattern surfaces, and techniques for learning "good" pattern features and shapes and flagging unfamiliar shapes encountered during inspection scanning as disclosed, for example, in U.S. Pat. Nos. 4,589,140 and 4,893,346, developed by the common assignee herewith and as incorporated first in the Model B-2000 of Beltronics, Inc. of Massachusetts and more recently in the Nikon Model AI1020 of Nikon of Japan. Among the variants in the latter techniques, as described in said patents, is the use of coded light, including color coding, to distinguish transmitted from reflected light useful particularly with via or other holes or apertures in the circuit board or other object or surfaces being inspected.
More recently, a very different approach to inspection processes, particularly useful with wafers (and other types of surfaces or objects having similar types of characteristics), was applied in the Beltronics "Microscan" Model, using intelligent image shrink and expand technology to enable inspection of portions of the light-reflected images of the surfaces in the context of the nature or characteristics of the surrounding material of the surface, thus to provide increased inspection discrimination and reliability, as described in copending U.S. patent application Ser. No. 636,413, filed Dec. 31, 1990 for Method of and Apparatus for Geometric Pattern Inspection Employing Intelligent Image-Pattern Shrinking, Expanding and Processing to Identify Predetermined Features and Tolerances, now U.S. Pat. No. 5,119,434, issued Jun. 2, 1992.
All of the above systems, to one degree or another, have a certain measure of criticality in requiring uniformity or appropriate light illumination intensity for illuminating the surface or object to be inspected; and substantial variations in such illumination, invariably mitigate against the satisfactory operation of the inspection system and require careful adjustment and readjustment.
Underlying the present invention, however, is a discovery of a technique for remarkably obviating such problems and limitations, particularly where the surface or objects to be inspected are multicolored, as, for example, in such wafers, where different component parts appear as of different colors and hues. The invention, indeed, not only renders the inspection system far less susceptible to operational problems resulting from light illumination intensity variations, as occur in prior art systems, but synergistically significantly improves the discrimination capability of the inspection process, as well.